Triboelectric sprayer

ABSTRACT

The invention concerns a triboelectric sprayer for spraying electrostatic powder paint, comprising a powder intake, an air intake, a conduit transporting air and powder while providing the electrical charge of the powder and, in front of the charging conduit, means for emitting at least an air-powder mixture jet. The air intake is located in the axis of the charging conduit, while the powder intake is arranged laterally and converges in an acute angle with the air intake, in a point located upstream of the charging conduit. The air intake includes of an injector made of electrically conductive material which also constitutes an earthing member of the wall of the charging conduit.

BACKGROUND OF THE INVENTION

The present invention relates to a triboelectric spray gun for theelectrostatic spraying of powder paint.

The principle of a triboelectric spray gun for the electrostaticspraying of powder paint and the following.

The paint, in insulating powder form, conveyed by “transport” air, iselectrostatically charged by friction against an insulating material,capable of withdrawing electrons therefrom, so that the powder becomespositively charged.

As is generally known, a triboelectric spray gun operating according tothis principle comprises:

-   -   a powder intake,    -   an air intake,    -   a duct transporting the air and the powder, while ensuring that        the powder becomes electrically charged; and    -   in front of said charging duct means for emitting at least one        jet of air/powder mixture.

The insulating material, ensuring that the powder becomeselectrostatically charged by friction, forms the wall of theaforementioned duct that transports the air and powder. This insulatingmaterial is electrically connected to ground via joints betweencomponents of the spray gun, so that it remains overall electricallyneutral, its charge permanently flowing away by “creep” (flow over thesurface), to ground. Of course, the higher the friction force betweenthe powder and the walls of the charging duct, the greater or better theelectrical charging of the powder.

Many documents have already described this kind of triboelectric spraygun. As examples, mention may be made here of European PatentApplication EP 0 163 118, EP 0 199 054, EP 0 314 049, EP 0 592 137 andEP 0 818 245, or else International Patent Applications WO 88/08332, WO92/11950 and WO 98/25707.

Most of the triboelectric spray guns described in those documents have,as charging ducts, a relatively elongate annular space bounded by partsagainst which the powder transported by the air will rub and be charged.The annular charging duct is bounded internally by a central core thatadvantageously has, in profile, a wavy shape (see in particular theaforementioned documents EP 0 314 049 (FIG. 3), EP 0 592 127 and WO92/11950) so as to increase the stirring of the powder, and therefore tofacilitate its contact with the insulating material provided forcharging it. The central core is held in place inside an external body,by which the charging duct is bounded externally, by centering elementsmounted at each end of said duct. This central core may have a conicalend on the side on which the inlet of the charging duct lies, so as todistribute the powder in the annular space forming this duct; the otherend of the central core, located on the side on which the outlet of thecharging duct lies, may also be of conical shape, so as to collect thecharged powder before it is sent into small outlet channels belonging toor associated with the means for emitting jets of air/powder mixture,these means being in practice formed by a nozzle.

As is usual, in the rear part of the triboelectric spray gun, an airintake is formed along the central axis of this spray gun. However, inmost known constructions, an additional intake for air, and if necessaryfor powder, is placed so as to be orthogonal to the central axis of thespray gun, upstream of the entry cone of the charging duct—see forexample the aforementioned documents EP 0 592 137 and EP 0 818 245. Thepresence of an obstacle-forming “orthogonal” injection means createsperturbations; in particular, it is the cause of premature wear and/orof powder building up at the stagnation point.

Moreover, in current triboelectric spray guns, the grounding, especiallyof the central core of the charging duct, requires the addition of partsthat complicate the set-up; this grounding is generally made around theexternal periphery and toward the rear of the body of the spray gun.Owing to the electrical forces and fields employed by the chargescreated, it is frequently observed that arcing occurs between the innerportion of the spray gun and its grounding region, and this leads toparts that are not, per se, wear parts having to be replaced relativelyfrequently.

BRIEF SUMMARY OF THE INVENTION

The present invention aims to avoid all these drawbacks of currenttriboelectric spray guns, by providing an air and powder intake systemthat avoids any wear and powder build-up, while simplifying thegrounding.

For this purpose, provision is made, in the triboelectric spray gunforming the subject of the invention, of the type indicated in theintroduction, for:

-   -   the air intake to be located on the axis of the charging duct;    -   the powder intake to be placed laterally and to converge, at an        acute angle to the air intake, at a point located upstream of        the charging duct.

Thus, any injection member forming an obstacle to the flow iseliminated, the air and powder intakes converging, at an acute angle,before the air/powder mixture reaches the turbulent charging region.

Advantageously, the powder intake, converging with the air intake, isformed by a powder duct that emerges on the rear face of the spray gun,thereby reducing the size and making the connections easier.

According to one embodiment of the triboelectric spray gun forming thesubject of the invention, the charging duct is a duct whose circularcross section is not constant, having successive restrictions, the pointof convergence of the powder and air intakes being located upstream ofthe first restriction in the charging duct. The choice of such acharging duct may have advantages compared with a charging duct ofannular cross section, especially as regards a certain constructionalsimplification (the central core is eliminated) and an improvement inthe operation (the risk of the annular space being blocked by the powderis eliminated).

However, the invention remains compatible with a triboelectric spray gunin which the charging duct is a duct of annular cross section, having acentral core with a rear cone, in which case the point of convergence ofthe powder and air intakes is located upstream of the apex of said cone.

In all cases, cross sectional restrictions provided along the chargingduct make it possible to create regions of impact by the powder on theinsulating material, thereby charging it. More particularly, if thecharging duct has an annular cross section, this annular cross sectionis not constant, it having successive restrictions.

In addition, a throttling region is advantageously provided downstreamof the charging duct, said throttling region terminating in ahomogenizing chamber from which the means for emitting at least one jetof air/powder mixture are fed. The latter restriction makes it possibleto accelerate the air/powder mixture so as to put the powder back into ahomogeneous suspension before it is delivered to the outlet channels.

According to another aspect of the invention, the axial air intake isformed by an injector made of an electrically conducting material, thusforming a grounding member. This arrangement makes it possible tosimplify the grounding function, the electric charges picked up from thepowder flowing away through the air injector.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood with the aid of thedescription that follows, with reference to the appended schematicdrawings showing, by way of examples, a few embodiments of thistriboelectric spray gun:

FIG. 1 is a side view, from outside, of a triboelectric spray gunaccording to the present invention;

FIG. 2 is an end view, from the rear, of the triboelectric spray gun ofFIG. 1;

FIG. 3 is a longitudinal sectional view of this spray gun of III—III inFIG. 2;

FIG. 4 is another longitudinal sectional view, on IV—IV in FIG. 2; and

FIG. 5 is a longitudinal sectional view, similar to FIG. 3, of analternative embodiment of this triboelectric spray gun.

BRIEF DESCRIPTION OF THE INVENTION

The triboelectric spray gun shown in FIGS. 1 to 4 and denoted overall bythe reference 1 comprises a main body 2, of tubular general shape with acentral axis indicated at 3, in which an elongate block 4 of insulatingmaterial is placed, defining a charging duct 5 centered on the axis 3.In the example in question, the charging duct 5 is of circular crosssection.

Provided at the rear of the main body 2 is an injection base 6,mechanically coupled to said body 2 via a screwable linking sleeve 7,and with the interposition of centering pins 8. The injection base 6 isitself mechanically coupled, via a fastening pin 9, to the end of amovable arm 10 (shown in part) belonging to a painting robot, the spraygun 1 being carried and moved by the robot arm 10.

In the junction region between the charging duct 4, 5 and the base 6,and lying along the central axis 3, there is an air injector 11. Thisinjector 11 is made of a material that is electrically conducting andalso has a high friction coefficient, such as graphitizedpolytetrafluoroethylene (PTFE), and it is grounded. For this purpose, agrounding screw 12 is provided in the base 6, the head of the screwcoming into contact with a flange of the air injector 11 (see FIG. 4).

An air/powder mixture intake fitting 13 is also mounted in the injectionbase 6, parallel to the central axis 3, and therefore in a lateralposition with respect to the air injector 11. The fitting 13 thusemerges on the rear face 14 of the base 6.

The fitting 13 is extended, to the front, by a short oblique channel 15that converges, at an acute angle A, at a point P, on the inlet of thecharging duct 4, 5 just downstream of the air injector 11.

Thus, during operation, the air injector 11 makes it possible to stirthe air/powder mixture that emerges in the inlet of the charging duct 4,5 via the oblique channel 15. The assembly allows a rapid and turbulentflow to be created in the charging duct 4, 5, preventing any laminarflow that would have the effect of reducing the friction needed forelectrostatically charging the powder.

In addition, restrictions 16 placed in succession along the chargingduct 4, 5, that is to say reductions in the diameter, and therefore inthe circular cross section of this duct, create regions of impact by thepowder which improve the friction of the latter on the material of theblock 4 defining the actual charging duct 5.

At the downstream end of the charging duct 4, 5 there is an outlet cone17 that makes it possible to deliver the air/powder (now charged) intooutlet channels 18 provided in an ejection nozzle 19 fitted in front ofthe spray gun 1. The nozzle 19 itself receives an outlet plug 20, theconfiguration of which depends on the type of jet desired at the outletof the spray gun, for example a flat jet.

FIG. 5, in which the components corresponding to those described aboveare denoted by the same references, shows an alternative embodiment ofthe triboelectric spray gun 1 that is the subject of the invention.

In particular, in this alternative embodiment, there are still providedan air injector 11, located on the central axis 3 of the spray gun 1,and an air/powder mixture intake fitting 13 placed laterally andextended by an oblique channel 15, which converges at a point P with theoutlet of the air injector 11, making an acute angle A with the axis 3.

Unlike the embodiment shown in FIGS. 1 to 4, the embodiment in FIG. 5has a charging duct 5 of annular (and not circular) cross section, thisbeing bounded on the outside by a tube 21 and on the inside by a centralcore 22, the tube 21 and the central core 22 being coaxial and centeredon the axis 3.

The central core 22 has a rear cone 22 a located downstream of the pointP where the oblique channel 15 converges with the outlet of the airinjector 11.

The charging duct 5 has, here again, successive restrictions (notdetailed in FIG. 5) that give it an annular cross section that decreasesfrom the upstream end toward the downstream end.

The central core 22 also has a front cone 22 b, surrounded by athrottling region 23 that extends the charging duct 5, with a decreasingcross section, and that emerges in a central homogenizing chamber 24.Downstream of this homogenizing chamber 24 there is an outlet cone 17that delivers the air/powder (now charged) mixture into outlet channels18 of an ejection nozzle 19 which is itself provided with an outlet plug20, for example designed to emit a flat jet.

It should be noted that, in this embodiment, the grounding of the tube21, by which the charging duct 5 of annular cross section is bounded onthe outside, is accomplished by means of a contact blade 25.

In all cases, unscrewing the linking sleeve 7 allows the base 6 to berapidly removed, especially for cleaning purposes, this sleeve 7 beingmounted with the interposition of O-ring seals (not detailed).

The number and the arrangement of the pins 8 and 9, or the configurationof the outlet plug 20, can be modified without thereby departing fromthe scope of the invention, as defined in the claims appended hereto.

1. A triboelectric spray gun for the electrostatic spraying of powderpaint, of the kind comprising: a powder intake; an air intake; acharging duct transporting air and powder, while ensuring that thepowder becomes electrically charged; in front of said charging duct,means for emitting at least one jet of air/powder mixture; wherein, theair intake is located on an axis of the charging duct; and the powderintake is placed laterally and converges, at an acute angle to the airintake, at a point located upstream of the charging duct and downstreamof the air intake.
 2. The triboelectric spray gun as claimed in claim 1,wherein the powder intake, converging with the air intake, is formed bya powder duct that emerges on a rear face of the spray gun.
 3. Thetriboelectric spray gun as claimed in claim 1, wherein the charging ductis a duct whose circular cross section is not constant, havingsuccessive restrictions, the point of convergence of the powder intakesand air intake being located upstream of a first restriction in thecharging duct.
 4. The triboelectric spray gun as claimed in claim 1,wherein the charging duct is a duct of annular cross section, having acentral core with a rear cone, the point of convergence of the powderintakes and air intake being located upstream of an apex of said cone.5. The triboelectric spray gun as claimed in claim 4, wherein theannular cross section of the charging duct is not constant, havingsuccessive restrictions.
 6. The triboelectric spray gun as claimed inclaim 1, wherein the axial air intake is formed by an injector made ofan electrically conducting material, thus forming a grounding member. 7.The triboelectric spray gun as claimed in claim 1, wherein a throttlingregion is provided downstream of the charging duct, said throttlingregion terminating in a homogenizing chamber from which the means foremitting at least one jet of air/powder mixture are fed.
 8. Atriboelectric spray gun for the electrostatic spraying of powder paint,of the kind comprising: powder intake; an air intake; a charging ducttransporting air and powder, while ensuring that the powder becomeselectrically charged; in front of said charging duct, means for emittingat least one jet of air/powder mixture; wherein, the air intake islocated on an axis of the charging duct; and the powder intake is placedlaterally and converges, at an acute angle to the air intake, at a pointlocated upstream of the charging duct; and the powder intake, is formedby a powder duct that emerges on a rear face of the spray gun.